1. Field of the Invention
The present invention relates to a high efficiency light emitting device, and more particularly to a III-V compound semiconductor light emitting diode with a highly reflective metal reflector therein to avoid the light absorption by the substrate.
2. Description of the Prior Art
The conventional AlGaInP LED, as shown in FIG. 1, has a double heterostructure (DH), which is consisted of an n-type (AlxGa1-x)0.5In0.5P lower cladding layer 3 with an Al composition of about 70%-100%, formed on an n-type GaAs substrate 1, an (AlxGa1-x)0.5In0.5P active layer 5 with an Al composition of 0%-45%, a p-type (AlxGa1-x)0.5In0.5P upper cladding layer 7 with an Al composition 70%-100% and a p-type high energy bandgap current spreading layer 9 such as layers of GaP, GaAsP, AlGaAs or ZnO. However, the portion of the light emits from the active layer 5 towards the substrate will be totally absorbed by GaAs substrate 1. Therefore, the external quantum efficiency of this kind of conventional AlGaInP LED is small. Besides, the thermal conductivity of GaAs is only about 44 W/m-° C. The low thermal conductivity of the GaAs substrate 1 is not good enough to dissipate the heat generated.
To overcome the substrate absorption problem, several conventional LED fabrication technologies have been disclosed. However, those conventional technologies still have several disadvantages and limitations. For example, Sugawara et al. disclosed a method published in Appl. Phys. Lett. Vol. 61, 1775-1777 (1992), The LED structure is similar to the FIG. 1, thus, in FIG. 2, the similar function layers are labeled with the same reference numerals. Sugawara et al. added a distributed Bragg reflector (DBR) layer 2 in between the GaAs substrate 1 and lower cladding layer 3 so as to reflect those light emitted toward the GaAs substrate 1, as shown in FIG. 2. Further they added a blocking layer 10 to enhance current spread. However, the maximum reflectivity of the DBR layer 2 used in AlGaInP LED is only about 80% and the reflectivity thereof also depends on the reflection angle. The DBR layer 2 can only effectively reflect the light vertically emitted towards the GaAs substrate 1, so that the improvement of external quantum efficiency is limited.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1-x)0.5In0.5P/GaP light emitting diode [Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994); Very high efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1-x)0.5In0.5P/GaP]. As shown in FIG. 3, a transparent-substrate 13 (TS) is replaced for the GaAs absorption substrate (not shown). The TS AlGaInP LED was fabricated by growing a very thick (about 50 um) p-type GaP window layer 11 formed on epi-layers light emitting structure 12 (0.75 mm p-type cladding layer 3 of Al0.5In0.5P/active layer 5 of AlxGa1-x)0.5In0.5P/1 mm n-type cladding layer 7 of Al0.5In0.5P with GaAs as temporary substrate by using hydride vapor phase epitaxy (HVPE). Subsequently, the temporary n-type GaAs substrate was selectively removed using conventional chemical etching techniques. After removing the GaAs substrate, the LED epilayer structure 12 is then bonded to an 8-10 mil thick n-type GaP substrate 13. The resulting TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LEDs. However, the fabrication process of the TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology [Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding][J Electronic Materials, Vol. 30, No.8, 2001, 907; Wafer bonding of 50-mm-diameter mirror substrates to AlGaInP light-emitting diode wafers]. Please refer to FIG. 4A, They used the AuBe 23/Au 21 of about 100 nm/150 nm in thickness as a mirror layer and adhered to SiO2 25/Si substrate 27 to form a mirror substrate 30. The LED epi-layers 20 is shown in FIG. 4B, which is similar to that shown in FIG. 2, but has a GaAs buffer layer 2a in between an n-type GaAs substrate 1 and an n-type DBR layer 2 of AlGaAs/GaAs, and a p-type GaAs capping layer 15 replaces for current spreading layer 9. The mirror substrate 30 is then binded with the LED epi-layers 20 by bonding the Au layer 21 with p-type capping layer 15. After that, the GaAs substrate 1, the GaAs buffer layer 2a, and the DBR layer 2 are removed. Finally an n-type electrode of AuGeNi/Au metal layer 19 is formed on the n-type cladding layer 3. The resulting structure is shown in FIG. 4C.
The purpose of the mirror substrate 30 is to reflect the light emitted towards the absorption substrate and to provide a better thermal conductivity silicon substrate. The silicon of mirror substrate 30 has a thermal conductivity of about 124-148 W/m-° C., and thus it can improve the heat dissipation problem. However, the top surface of AlGaInP LED epi-wafer 20 normally has some hillocks (not shown). These hillocks can result in incomplete bonding regions while the LED epi-layers portion 20 bonds with the supporting substrate 30 these regions will be problematic and present deteriorated performance of LED chips. Moreover, to achieve lower contact resistance, the n-type ohmic contact 19 must be annealed at a temperature higher than 400° C. At such a higher temperature annealing, the reflectivity of the Au mirror layer 21 may seriously degrade because of the reaction between Au layer 21 and the III-V compound semiconductor: the p-type GaAs capping layer 15. Besides, both p-electrode 21 and n-electrode 19 are formed on the same side, so that the chip size is larger than conventional LED chip that has p-electrode on one side and n-electrode on the other side.
Chen et al. in U.S. Pat. No. 6,319,778 B1 disclosed a light emitting diode with metal reflector to increase the light output. The LED structure is shown in FIG. 5, is composed of a LED epi-layers 40 and a supporting substrate 35 bonded by a low temperature solder layer 39. The LED epi-layers 40 is consisted of an n-type cladding layer 41, an AlGaInP active layer 42, a p-type cladding layer 43, a p-type GaAs capping layer 44 and a p-type ohmic contact layer 45. The supporting substrate 35 is comprised an impurity heavily doped silicon substrate 36 coated with metal layers 37a and 37b on both sides of the silicon substrate 36. Therefore, a vertical injection current flow LED structure with n-electrode on one side (an n-type ohmic contact metal 47 and p-electrode on the other side 37a) can be achieved. However, the n-ohmic contact metal 47 is deposited after bonding. To achieve lower contact resistance, a high temperature annealing process is necessary but will degrade the reflectivity of metal reflector 37b. In order not to sacrifice the reflectivity, the n-ohmic contact metal 47 can't be annealed in higher temperature. Therefore, a lower n-type ohmic contact 47 resistance can't be achieved.